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Python enhanced assignment and shared reference considerations

The enhanced assignment in Python is borrowed from the C language, so most of these formats are the same as C, which is the shorthand of expressions, that is, the combination of binary expressions and assignment statements, such as a + = b and a = a + b, for example, there are also the following enhanced assignment statements.

That is, enhanced assignment statements apply to any type that supports implicit binary expressions, such as the polymorphism of "+": the addition of numbers and the merging of strings.

Digital addition and subtraction

A = 1a = a + 1print (str (a)) a + = 1print (str (a))

Example result:

twenty-three

String merging

S = 'Python.'print S = S +' like 'print (S) S + =' Python.'print (S)

Example result:

I likeI like Python.

Advantages

Brevity

Reduce the execution of a, and the execution speed is faster

For mutable objects, enhanced assignment automatically chooses to perform the modification operation in place, rather than a slower copy. This leads to the problem of shared references that we may involve in mutable objects.

Shared reference

What do we do when we want to expand the list, such as adding a set of elements to the end?

L = [1,2,3] # traditional "+" method L = L + [4,5] print (L) # using list method extendL.extend ([6,7]) print (L)

Sample result

[1, 2, 3, 4, 5] [1, 2, 3, 4, 5, 6, 7]

In the first example, the "+" method, that is, using the merge method, requires the creation of a new object to copy the L on the left to the new list, and then [4, 5] to the new list. The second kind of extend is to add [4,5] directly to the end of the memory space list L, that is, the speed will be faster, and the enhanced assignment is automatically adopted, that is, L.extend ([6,7]) and L + = [6,7] are equivalent and the best choice. Although this kind of merge is fast, the shared reference to mutable objects becomes trickier.

L1 = [1,2,3] L2 = L1L2 = L2 + [4,5] print (L2) print (L1) print ('-'* 21) L1 = [1,2,3] L2 = L1L2 + = [4,5] print (L2) print (L1)

Example result:

[1,2,3,4,5] [1,2,3]-[1,2,3,4,5]

We can see from the example that if multiple variables are assigned to the same mutable object, then when breaking the shared reference structure, the mutable object should be copied and manipulated.

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